1. Field of the Invention
This invention relates to the treatment of gaseous streams containing appreciable quantities of an inert gas, such as nitrogen. It more closely relates to removing and recovering methane and higher boiling hydrocarbons from a gas stream, natural or synthetic, which contains large quantities of nitrogen or hydrogen, may contain acidic components such as CO.sub.2 and H.sub.2 S, and may vary in moisture content from dry to saturated. It specifically relates to the removal of inert gases, such as nitrogen, from a gas stream in order to upgrade its heating value. It more specifically relates to adapting the extractive stripping embodiment of the Mehra Process for processing of gas streams containing hydrocarbons and contaminated with an inert gas. It additionally relates to purification of the lean solvent stream.
2. Review of the Prior Art
Many natural gases are contaminated with one or more inert gases which lower their Btu content or otherwise impair their marketability. Such inert gases include nitrogen, helium, argon, and, under ambient conditions, hydrogen in combination with alkanes and similar gaseous compounds of low reactivity. Refinery and petrochemical gaseous treatment products, containing relatively large amounts of hydrogen and highly reactive compounds such as olefins, are specifically excluded from the gases hereinafter under consideration.
During recent years, there has been strong emphasis on the secondary and tertiary methods of recovering oil from formations where the primary oil-producing methods are no longer productive. Nitrogen injection for reviving these oil wells is not useful in most formations, but in some formations such as in the central Texas area, nitrogen injection has been successfully utilized for the recovery of additional oil.
After several years of nitrogen injections at high pressure, approximately 2000 psig, the nitrogen seems to have broken through the formations in many instances. In other words, nitrogen is coming out with the oil and it is separated from the oil at the separator. Previously, the associated gases were rich in hydrocarbons heavier than methane, along with substantial quantities of methane. The present dilution effect of nitrogen has caused the same associated wellhead gas to have an extremely low BTU content, thereby making it unsuitable for pipeline shipments. If the natural gas contains more than 3% of nitrogen, it is off specification for most of the world's pipelines.
This situation has caused the oil producer to curtail oil production because he cannot burn the nitrogen-rich gas, and environmental laws prohibit him from venting the associated hydrocarbons. The oil producer is thus limited to the choice of technology available to him for properly processing the associated gases from an oil well. The available technology involves cryogenic principles, thereby causing the purified gas to be uneconomical in the natural gas market, even after subsidization with the revenues from oil production.
Natural gas is a mixture of hydrocarbons, including methane, ethane, propane, and various amounts of higher molecular weight hydrocarbons together with nitrogen and acid gases, such as CO.sub.2 and/or H.sub.2 S. A "dry" gas is one containing predominantly methane with some ethane, propane, and butane and having a very low hydrocarbon dew point. The heavier the hydrocarbons, such as pentane and higher homologs, that are present in the gas, the higher the hydrocarbon dew point. For pipeline transmission, enough of the heavier hydrocarbons must be removed to lower the dew point without losing too many BTUs to meet specifications. In the past, gas with large quantities of high molecular weight hydrocarbons have been passed through gasoline extraction plants and/or dew point control stations to lower the dew point. Also, frequently the gas has required conditioning to remove sulfur compounds and carbon dioxide.
A nitrogen-rich gas stream, which can vary in composition from 3 to 75 mol. % nitrogen, the remainder being hydrocarbons, and from entirely dry to water saturated and from sweet to sour, can be extracted according to the extractive flashing embodiment of the Mehra Process in at least one and, if necessary, up to eight gas-extracting steps with a preferential physical solvent to provide up to three products, namely: a nitrogen gas product, a C.sub.1 -rich gas product, and a C.sub.2 + liquid product, as taught in U.S. Pat. No. 4,623,371, which is incorporated herein by reference. The process upgrades the Btu value of a nitrogen-rich natural gas stream, for example, by extracting C.sub.1 + hydrocarbons from the gas and then selectively separating the extracted products from the rich solvent by flashing to produce a C.sub.1 rich gas stream with minimal amounts of nitrogen and a C.sub.1 -lean gas stream which is compressed, cooled, and condensed and finally demethanized to provide the C.sub.2 + liquid product.
In addition to separation of methane from nitrogen, this extractive flashing embodiment of the Mehra Process provides selective recoveries of ethane in amounts ranging from 2-98%, propane in amounts ranging from 2-99%, butanes in amounts ranging from 2-100%, and pentanes and higher molecular weight hydrocarbons in amounts ranging up to 100%. Under the heading, "New NGL Extraction Process", the extractive flashing embodiment of the Mehra Process is described on pages 7 and 8 of the Oct. 14, 1985 issue of the Gas Processors Report, P.O. Box 33002, Tulsa, Okla. 74153. However, profitability for this embodiment of the Mehra Process can be improved by simplifying process design and minimizing capital and maintenance costs. Such a process has been disclosed in U.S. Pat. No. 4,617,038 and Ser. No. 808,463 which are incorporated herein by reference.
This improvement comprises: (a) selectively extracting and then stripping the natural gas stream with a preferential physical solvent to produce a natural gas stream of pipeline specifications and a rich solvent stream containing ethane and heavier hydrocarbon components and then (b) distilling the rich solvent to produce the natural gas liquids and the physical solvent for recycling to the extractive stripping step.
There remains, nevertheless, a need for processing an inert-rich gas stream to provide an inert gas product, a C.sub.1 -rich gas product, and a selectively extracted C.sub.2 + liquid product. The processes of Ser. No. 784,566 and of Ser. No. 808,463 are not designed for meeting this need.
The processes of the parent U.S. Pat. Nos. 4,421,535; 4,511,381; and 4,526,594; all of which are incorporated herein by reference, utilize preferential physical solvents for processing natural gas streams by extracting, flashing, compressing, cooling, and condensing the desired components for producing natural gas liquid products. Ser. No. 759,327 is particularly directed toward processing of nitrogen-rich natural gas streams in this manner. In contrast, the process of Ser. Nos. 784,566 and 808,463 utilizes an extractive stripping (ES) step and minimizes the need for flashing of the rich solvent stream to separate the desired components of a raw gas stream.
Extractive distillation is well known in the prior art and is characterized by condensation of the overhead stream and refluxing of at least a portion of the condensed materials therein. Extractive stripping, in contrast, utilizes no condensation of the overhead stream and instead has a complete flow through of gaseous and liquid products, without reflux.
The preferential physical solvents preferred for the process of Ser. No. 808,463 are rich in monocyclic C.sub.8 -C.sub.10 aromatic compounds having methyl, ethyl, or propyl aliphatic groups and selective for ethane and heavier hydrocarbons components of the gas stream such that: (a) the minimum relative volatility of methane over ethane is at least 5.0 (thereby defining its improved selectivity toward ethane over methane) and in addition a solubility of ethane in the solvent of at least 0.25 standard cubic foot of gaseous hydrocarbons per gallon of the solvent (SCF/GAL) (thereby defining its hydrocarbon loading capacity), or, alternatively, a preferential factor of at least 1.25. The preferential factor for physical solvent selection is defined as a product of relative volatility of methane over ethane multiplied by the solubility of ethane in physical solvents, specified as standard cubic feet of ethane per gallon (SCF/gal). However, the ideal preferential physical solvent would have a selectivity toward ethane over methane of at least 10.0 and would simultaneously possess a hydrocarbon loading capacity of at least 3.0 SCF/GAL. This combination of minimum relative volatility and minimum solubility enables solvent flow rate variations and operating pressure variations to be selectively utilized for flexibly producing liquid products having selected hydrocarbon compositions.
U.S. Pat. No. 2,325,379 teaches a process for separating a liquid mixture of components by extractive distillation in the presence of a relatively high boiling selective solvent which may be a polar solvent.
U.S. Pat. No. 2,357,028 relates to extractive distillation of a liquid mixture with a highly selective solvent, such as phenol, furfural, sulfolane, toluene, xylene, and ethyl benzene. A volatility ratio or "alpha value" is defined and given as a direct measure of the selectivity of the solvent.
U.S. Pat. No. 2,433,286 is directed to extractive distillation of liquid hydrocarbon mixtures with paraffin hydrocarbons as the extraction solvent in a first extractive distillation to produce olefins plus diolefins in the rich solvent and in a second extractive distillation with unsaturated or aromatic hydrocarbons as the solvent at a higher temperature to produce olefins as the raffinate and diolefins in the rich solvent. Paraffins are distilled from the rich solvent of the first extractive distillation and diolefins are distilled from the rich solvent of the second extractive distillation.
U.S. Pat. No. 2,455,803 describes a process for extractive distillation of a vaporizable organic mixture with a solvent comprising (1) a selective solvent and (2) a mutual solvent for the selective solvent and the mixture. The selective solvent must have high selectivity which is frequently coupled with low solvent power, thereby tending to form two liquid layers within the extractor. The purpose of the mutual solvent is to maintain a single liquid phase. The presence of the solvents in the mixture must cause a greater change in the "escaping tendency" of one component of the mixture relative to that of the other components, "escaping tendency" being defined as the potential of one component to pass from one phase to another. Solvents such as furfural and phenol are named as those having preferential solvent power for aromatic over paraffinic hydrocarbons. Suitable mutual solvents are identified as methyl ketone, cyclohexanone, lactonitrile, morpholine, and aromatic hydrocarbons such as benzene, toluene, cumene, mesitylene, and the like.
U.S. Pat. No. 2,559,519 relates to fractionating a liquid mixture of close-boiling oxygenated compounds in the presence of a large excess of a glycol-ether by continuous fractional distillation in a column of practical size, including a primary rectification zone, a secondary rectification zone above the primary zone, and a stripping zone below the primary zone for countercurrent vapor-liquid contact under reboiling and refluxing conditions.
U S. Pat. No. 2,570,066 is directed to a method of segregating pure hydrocarbons from hydrocarbon mixtures by extractive distillation in the presence of an aromatic hydrocarbon solvent which is preferably a mono-cyclic aromatic hydrocarbon fraction boiling in the range between 365.degree. and 750.degree. F. Mono-cyclic aromatic hydrocarbons having 10 carbon atoms, exemplified by tetramethylbenzenes such as 1,2,4,5-tetramethylbenzene, 1,2,3,5-tetramethylbenzene, and 1,2,3,4-tetramethylbenzene, and further exemplified by 1,2,-dimethyl-3-ethylbenzene, 1,2-dimethyl-4-ethylbenzene, and the like, are preferred. Durene, isodurene, prehnitene, and mixtures thereof are especially beneficial. The ratios of solvent to feedstock may range from about 1:1 to about 20:1, about 5:1 being preferred.
U.S. Pat. No. 3,280,206 relates to liquid-liquid extraction with inert organic solvents such as carbon tetrachloride, chloroform, tetrahydrofuran, diethylene glycol dimethylether, and benzenoid hydrocarbons which are free of olefinic and acetylenic unsaturation and boil at a temperature which is below the boiling point of the high boiler, such as benzene, toluene, ethylbenzene, xylenes, mesitylene, biphenyl, the lower alkyl biphenyls, and the terphenyls, in order to remove high boiling polyphenyls which have been formed by exposure to heat and/or ionizing radiation of organic coolants and coolant-moderators in nuclear reactors.
U.S. Pat. No. 3,616,271 teaches an extractive distillation method of separating chloroform and/or ethyl acetate from vinyl acetate by using a hydrocarbon having a boiling point of 100.degree.-250.degree. C. as the extractive solvent. Alpha values, as the ratios of relative volatilities determined from equilibrium distillation data for 1% solutions of chloroform and ethyl acetate in vinyl acetate, are calculated and used for evaluating the solvent. The greater the alpha value, the more volatile are liquids being removed as a substantially pure stream from the top of the column while the less volatile liquids are separated together with the extraction solvent from the bottom of the column. Among suitable solvents are alkyl aromatic hydrocarbons such as xylene, triethyl benzene, n-butyl benzene, and mesitylene.
Inert-rich natural gas streams can be processed according to the disclosures of U.S. Pat. No. 4,617,038 and Ser. No. 808,463, provided that it is acceptable to separate only the methane and heavier hydrocarbons (C.sub.1 +) from the gas streams and to reject the inert gas contained therein. Such a process is shown in FIG. 1, wherein the undesirable gases, consisting primarily of nitrogen, leave the top of the Extractor-Stripper (ES) column while the rich solvent containing the desired components leaves the bottom of the ES column. The C.sub.1 + hydrocarbons are then separated from the rich solvent as a gas stream (e.g., a suitable natural gas stream for sale before processing to remove selective C.sub.2 + components) from the top of the hydrocarbon product column while the separated solvent is recycled to the ES column for reuse.
The processes of Ser. Nos. 784,566 and 808,463 are not designed to separate the C.sub.2 + hydrocarbons from an inlet nitrogen-rich natural gas stream and additionally to Btu-upgrade the residue natural gas stream. There is therefore a need to provide a process that includes the benefits of the inert-gas isolating invention of U.S. Pat. No. 4,623,371 in U.S. Pat. No. 4,617,838 and Ser. No. 808,463, namely, lower capital and energy requirements along with simplification of process design, and that is also capable of processing nitrogen-rich gas streams according to desired economics of the market.
An example of pertinent market economics occurs under poor economic conditions when ethane price as petrochemical feedstock is less than its equivalent fuel price and when the propane price for feedstock usage is attractive. At such times, the operator of a natural gas liquid extraction plant, for example, is limited as to operating choice because he is unable to minimize ethane recovery and maximize propane recovery in response to market conditions. A process is therefore needed that would separate N.sub.2 from a fuel gas consisting essentially of methane and ethane and from an NGL product consisting essentially of C.sub.3 + hydrocarbons.
There is consequently a further need for an extractive stripping process wherein propane and heavier hydrocarbons can be extracted to any selected degree from a natural gas stream without the need to extract significant quantities of ethane. There is still further a need for an extractive stripping process wherein butanes and heavier components can be recovered to any selected degree from a natural gas stream at extremely high recoveries without the need simultaneously to recover propane and ethane from the natural gas stream. There is at times also a need for an extractive stripping process wherein pentanes and heavier hydrocarbons can be recovered to any selected degree from a natural gas stream at extremely high recoveries without the need simultaneously to recover ethane, propane, and butanes therefrom.
An additional problem that arises in such extractive stripping processes is caused by the presence of small amounts of cyclic compounds in the gas stream when the cyclic compounds have a higher boiling point (i.e., a higher molecular weight) than the solvent. Under such circumstances, the cyclic compounds tend to build up in the solvent and cause the solvent to lose its preferential characteristics. There is accordingly also a need to provide a process that can maintain the preferential nature of the solvent without interfering with the extractive stripping process.